The purpose is to characterize and explain age associated losses of muscle strength, understand the effects of age and gender on the exercise response to resistive training, and the examine the phyiological contributions of motor units to strength production. 1. Characterization of longitudinal strength changes in the BLSA. Over the past year, we have reported cross sectional and longitudinal changes in arm isometric muscle strength and power. Over a 25-year period, repeated measures were taken approximately every two years from men in the Baltimore Longitudinal Study of Aging (BLSA). The longitudinal measures covered an average 9.57 years, range 1-25 years. Both upper extremity strength and power were reduced in older as compared with younger subjects, though power declined to a greater extent than strength. Twenty five-year longitudinal analyses in men confirmed the declines observed cross-sectionally. Loss of muscle mass did not completely explain the age associated changes in strength. Changes in peripheral nerve function as assessed by nerve conduction velocity were found to a small, independent contribution to the loss. In a study of knee extension, we found that women show less strength loss during eccentric than concentric movements with increasing age. One contributing factor appears to be a greater ability of older women to store and utilize elastic energy. An abstract was also presented that showed in a longitudinal analysis from the BLSA that women who take estrogen at the menopause show less strength loss than women who do not take estrogen. 2. Comparison of exercise response to resistive strength training in young and old subjects. After 9 weeks of resistive strength training both young and old, women and men showed approximately a 25% to 35% increase in muscle strength based on 1 RM with younger subjects showing a greater increase. After 20 weeks of detraining, both age groups continued to be stronger than at baseline. The increase in strength was associated with muscle hypertrophy that was greater in men than women, but no difference based on age. The increases were not found to be associated with increases in muscle damage in either young or old men. Isometric strength did not significantly increase, but during isometric exertion, a change occurred in motor unit activation with smaller units being used to generate the same force levels after as compared to before training. 3. Examination of the motor unit and its relationship to muscle strength and exercise response. Decomposition-enhanced spike-triggered averaging was applied to the vastus medialis muscle to examine size distribution of surface-detected motor unit action potentials and firing rates at various voluntarily generated force levels. Amplitude increased linearly with increasing knee extension force. Firing rate showed small increases to 30% of maximal knee extension force, and rose more rapidly at higher force levels. Average motor unit amplitude and firing rate accounted for approximately 75% of the variance in muscle force generation. During fatiguing submaximal isometric force at 30% of maximal a progressive increase in motor unit amplitude occurred with a gradual decline in firing rate, consistent with the concept of muscle wisdom advocated by Marsden and Merton.